1. Field of the Invention
The present invention relates to an electrophotographic image formation method capable of forming high quality images by developing latent electrostatic images formed on a photoconductor with a two-component type developer comprising a carrier and a toner for an extended period of time, by preventing the photoconductor from being scratched by the carrier of the two-component type developer, which remains on the surface of the photoconductor after the development of latent electrostatic images, even though the amount of the remaining carrier is slight.
2. Discussion of Background
Inorganic photoconductive materials such as selenium (Se), amorphous silicon (a-Si), and cadmium sulfide (CdS) are conventionally used as electrophotographic photoconductors. Recently, however, organic photo-conductors comprising an organic photoconductive material are widely used because of the advantages of organic photoconductors over inorganic photoconductive materials that the cost is low, no pollution problems are caused, and the electrophotographic characteristics are excellent.
Organic photoconductors have a hardness in the range of about 20 to 40 kg/cm.sup.2 because of the organic photoconductive materials employed therein, and therefore the mechanical durability thereof is low. Therefore, when such an organic photoconductor is used without particular treatment in electrophotographic copying machines, or in image formation apparatus for laser printers, the organic photoconductor will have be exchanged with a new organic photoconductor whenever about 50,000 to 100,000 copies have been made by use of the organic photoconductor. Therefore, many proposals have been made for increasing the hardness of a top layer of such an organic photoconductor to improve the abrasion resistance thereof.
One of the representative proposals for improving the abrasion resistance of an organic photoconductor is to provide a protective thin layer with high hardness as an overcoat layer on a photoconductive layer of the organic photoconductor.
Such a protective layer must not impair the optical characteristics and electrical characteristics of the photoconductor and must have excellent mechanical and chemical characteristics.
One of the most suitable protective layer for an organic photoconductor for use in the present invention is an amorphous carbon film (i.e. a-C film), which is also referred to as diamond-like carbon film. This amorphous carbon film is fabricated by vacuum film formation methods, such as the plasma CVD method, the light CVD method, and the sputtering method, by use of a hydrocarbon gas such as methane, ethane, ethylene, butane or butadiene, together with any of hydrogen, oxygen, fluorine, and nitrogen gases when necessary.
When a photoconductive layer is coated with the above-mentioned a-C film, it is necessary that the temperature of a substrate for the photoconductive layer must not be above room temperature in view of the glass transition temperature (Tg) of the photoconductive layer. By the selection of an appropriate gas from the above-mentioned various gases at the formation of the protective layer, and by appropriately setting the conditions for the formation of the protective layer, it is possible to fabricate a protective layer with excellent electro-photographic characteristics and mechanical abrasion resistance.
In the above-mentioned organic photoconductors with hard protective layers, and inorganic photoconductors, some hard foreign materials occasionally enter between a photoconductor and a cleaning member (or cleaning blade) for removing a developer from the surface of the photoconductor.
If this takes place, scratches with a thickness of several micrometers to several ten micrometers are formed in the surface of the photoconductor. Such scratches formed in the photoconductor may produce abnormal images such as non-printed lines or black lines in copies, thereby lowering the image quality obtained, depending upon the value of the surface potential at the photoconductor, and shorten the life of the photoconductor.
More specifically, electrophotographic image formation is carried out by uniformly charging a photoconductor to a predetermined polarity (positive or negative), for instance, with a potential of 500 volts to 1200 volts, projecting an original image processed in the form of digital signals to the charged photoconductor to form a latent electrostatic image corresponding to the original image, and developing the latent electrostatic image to a visible toner image by a two-component developer comprising a toner and a carrier.
In the above development process, (1) when the potential of the charged photoconductor is excessively high, (2) when the difference in the potential between a light area and a dark area in the original image is excessive, (3) when ultramicro particles are contained in the carrier, or (4) when a carrier with deteriorated chargeability is contained in the developer, the carrier is deposited not only on the image area, but also on the background of the latent electrostatic image, or non-printed portions are formed in the developed toner image.
Furthermore, when the carrier is deposited on a photoconductor and is transported up to a cleaning section for cleaning the photoconductor after the development of the latent electrostatic image, and for instance, a rubber blade is used as a cleaning member in the cleaning section, the carrier enters between the cleaning blade and the photoconductor, and scratches the surface of the photoconductor. When the surface of the photo-conductor is scratched, the image quality obtained is lowered as mentioned previously.
The deposition of the carrier on the photoconductor can be decreased to some extent by decreasing the charging potential of the photoconductor, but there is a limit to the decreasing of the charging potential of the photoconductor in practice, so that the deposition of the carrier on the photoconductor cannot be completely avoided.